We assessed Acr3-EGFP sperm binding to normal and huZP2 rescue eggs in which human ZP2 replaces mouse ZP2, but remains uncleaved after fertilization. The observed de novo binding of Acr3-EGFP sperm to embryos derived from huZP2 rescue mice supports a 'zona scaffold' model of sperm-egg recognition in which intact ZP2 dictates a three-dimensional structure supportive of sperm binding, independent of fertilization and cortical granule exocytosis. Surprisingly, the acrosomes of the bound sperm remain intact for &#8805; 24 hours in the presence of uncleaved human ZP2 regardless of whether sperm are added before or after fertilization. The persistence of intact acrosomes indicates that sperm binding to the zona pellucida is not sufficient to induce acrosome exocytosis. A filter penetration assay suggests an alternative mechanism in which penetration into the zona matrix initiates a mechanosensory signal transduction necessary to trigger the acrosome reaction. [unreadable] [unreadable] The only documented modification of the zona pellucida that temporally correlates with the zona block in mice and humans is cleavage of ZP2 which results in an N-terminal peptide (30 kD) that remains covalently linked to the parental fragment via a disulfide bond(s). Fortuitously, ZP2 remains uncleaved in huZP2 rescue mice in which human ZP2 replaces endogenous mouse ZP2 and the presence of only two pronuclei in one-cell embryos, their progression to the two-cell stage in vitro, and the birth of live pups attests to an effective block to polyspermy. Even though sperm continue to bind after fertilization, the absence of excessive sperm in the perivitelline space suggests that part of this block is attributable to the zona pellucida. If correct, the presence of intact huZP2 following fertilization suggests that the zona block observed with the humanized zona pellucida may be independent of huZP2 cleavage. Thus, it appears that cleavage of ZP2 is not required to block post-fertilization sperm penetration of the zona pellucida.